unmerged export: the comments name the FLIGHT column they forgot

The header and the two in-function comments describing what the unmerged
MTZ carries still said "two columns" and "raw counts are I / LP * QE",
written before the flight-path term and its FLIGHT column were added.
The code, the test and docs/RUGNUX_INTEGRATION.md all say I / LP * QE *
FLIGHT; only these comments did not. Also drops the self-referential
note in Reflection.h about a comment having been overtaken, and states
the flight factor's symbols and the medium rather than assuming air.

No behaviour change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011GxZqDiFP3KqriBhNdcR56

common/Reflection.h

@@ -35,12 +35,14 @@ struct Reflection {
     // sensor is opaque or its material and thickness are unknown. It is carried BESIDE
     // prescaling_corr rather than inside it, because LP and detector response are two different
     // things and every file the field reads keeps them apart. It is one of the three factors whose
-    // product is the total deterministic correction - see flight_corr below, which arrived after
-    // this comment was written and made "the two" wrong. Defaulted to 1 so a reflection read from a
-    // file written before this existed is a no-op rather than a zero.
+    // product is the total deterministic correction, with prescaling_corr above and flight_corr
+    // below. Defaulted to 1 so a reflection read from a file written before this existed is a no-op
+    // rather than a zero.
     float qe_corr = 1.0f;
-    // The air in the sample-to-pixel flight path, exp(D/L * (1/cos(alpha) - 1)): always >= 1, and
-    // exactly 1 when the air correction is switched off. Carried beside the two above for the same
+    // The medium in the sample-to-pixel flight path, exp(D/L * (1/cos(alpha) - 1)) with D the
+    // normal-incidence distance, L the medium's attenuation length and alpha the angle of incidence
+    // on the detector: always >= 1, because an oblique reflection crossed more of the medium than
+    // one arriving head-on. Exactly 1 under --flight-path vacuum. Carried beside the two above for the same
     // reason they are carried apart - it is neither beam geometry nor detector response but the
     // medium in between, and unlike either of them it is set by the flight distance. The total
     // deterministic correction on a reflection is prescaling_corr * qe_corr * flight_corr, and every

image_analysis/WriteReflections.cpp

@@ -733,7 +733,7 @@ void WriteUnmergedMtzReflections(const std::vector<IntegrationOutcome> &outcomes
     ds.wavelength = experiment.GetWavelength_A();
 
     // Every data column belongs to HKL_base and only the batches carry a dataset id, as in a
-    // POINTLESS file. Down to FLAG this is POINTLESS's own column set, plus QE; the four after it
+    // POINTLESS file. Down to FLAG this is POINTLESS's own column set, plus QE and FLIGHT; the four after it
     // are what rugnux measures beyond it - the offset of the reflection from the centre of its
     // rocking curve, the Lorentz geometry of that curve, and the background that was subtracted.
     //
@@ -787,10 +787,11 @@ void WriteUnmergedMtzReflections(const std::vector<IntegrationOutcome> &outcomes
     // crystal-frame information careless scales on, and what makes the file unmerged rather than a
     // merge waiting to happen.
     // I and SIGI are the integrated intensity with every deterministic per-reflection correction
-    // applied and nothing else, which is what IOBS means in every unmerged format. Two columns
-    // record what was applied, so the raw count sum is recoverable exactly as I / LP * QE:
-    // LP is Lorentz x polarization and QE the sensor efficiency (a divisor, hence the multiply).
-    // Neither is a scale, and a program that reads this file has no way to recompute them. The
+    // applied and nothing else, which is what IOBS means in every unmerged format. Three columns
+    // record what was applied, so the raw count sum is recoverable exactly as I / LP * QE * FLIGHT:
+    // LP is Lorentz x polarization, QE the sensor efficiency and FLIGHT the attenuation of the
+    // flight-path medium (both divisors, hence the multiplies).
+    // None of the three is a scale, and a program that reads this file has no way to recompute them. The
     // partiality is NOT divided out - that IS a scale, FRACTIONCALC carries it, and every program
     // this file is for wants to handle it its own way.
     gemmi::UnmergedHklMover hkl_mover(mtz.spacegroup);

image_analysis/WriteReflections.h

@@ -56,10 +56,11 @@ void WriteShelxHklReflections(const std::vector<MergedReflection> &reflections,
 // rotation program writes, over the events the combine would assemble (--min-partiality). False
 // writes one row per integrated box instead, flagged as partials for the reading program to sum.
 // Stills have no rocking events and are unaffected either way.
-// The intensities carry the deterministic per-reflection corrections and nothing else - Lorentz,
-// polarization and the sensor's angle-dependent efficiency, recorded in the LP and QE columns, so
-// raw counts are I / LP * QE. The partiality and the per-image scale are left for the reading
-// program to fit, since every program this file is for fits a scale model of its own.
+// The intensities carry the deterministic per-reflection corrections and nothing else - Lorentz
+// and polarization in LP, the sensor's angle-dependent efficiency in QE, and the attenuation of the
+// flight-path medium in FLIGHT - so raw counts are I / LP * QE * FLIGHT. The partiality and the
+// per-image scale are left for the reading program to fit, since every program this file is for
+// fits a scale model of its own.
 void WriteUnmergedMtzReflections(const std::vector<IntegrationOutcome> &outcomes,
                                  const UnitCell &unitCell,
                                  const DiffractionExperiment &experiment,